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-us

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The Esperanto suffixes -as, -is, -os, -us are related, and appear to have been inspired by previous language projects:

This play of vowels is not an original idea of Zamenhof's: -as, -is, -os are found for the three tenses of the infinitive in Faiguet's system of 1765; -a, -i, -o without a consonant are used like Z's -as, -is, -os by Rudelle (1858); Courtonne in 1885 had -am, -im, -om in the same values, and the similarity with Esperanto is here even more perfect than in the other projects, as -um corresponds to Z's -us. —An International Language (1928)

(diminutive suffix): It creates diminutive or affectionate forms, most of the time of people's given names, but also of common nouns, usually ending in a. Other examples: Anna → Annus, Magda → Magdus, Gyula → Gyulus. The -ka diminutive suffix can also be appended after -us to further increase the degree of endearment: apuska, cicuska, kutyuska, Annuska, Magduska.

See Appendix:Old Irish affixed pronouns for details on how these forms are used.
Note that the so-called “infixed” pronouns are technically prefixes, but they are never the first prefix in a verbal complex.